The present invention relates to an inker for use in a printing press and, in particular, relates to an inker for applying ink to a lithographic printing plate which has ink-receptive and ink-rejecting areas.
An inker for a lithographic printing press includes an ink fountain having a rotatable fountain roller and a doctor blade. Ink keys adjust the space between the fountain roller and the doctor blade to control the thickness of the ink film transmitted to the fountain roller. The ink is transferred from the fountain roller by a plurality of ink distribution rollers. The ink distribution rollers transmit the ink to a plurality of form rollers which apply the ink film to the printing plate. The ink distribution rollers include vibrating rollers which oscillate axially as they rotate.
A major ink starvation problem has been encountered in the printing of certain images by lithographic printing presses having an inker as noted above. The problem centers around the printing of images in the same color and shade where the image includes at least two portions, one portion which extends angularly relative to the other portion, such as where the image is, in effect, a picture frame such as printed on the cover of magazines, or an L or U or the like.
Typically, such an image is printed by directing the material to be printed through the press so that, for example, one side of the picture frame or one leg of the L is parallel to the direction of movement of the material through the press, and the other leg of the L or another side of the picture frame extends perpendicular to the direction of movement of the material through the press. Thus, the printing plate has abutting ink-receptive areas which extend circumferentially around the plate cylinder different distances and extend axially different distances. Hence, adjacent circumferential sections of the printing plate have different percentages of ink-receptive areas.
The ink starvation problem that has been encountered in the printing of such images is that one portion of the image will not be the same shade as the other portion of the image, and a very sharp distinct change in color occurs in the image which change in color is very visible to the human eye. Typically, the portion of the image which is printed by the ink-receptive plate area of the greatest circumferential extent around the plate cylinder (the circumferential section of the plate having the greatest percentage of ink-receptive area) is lighter in shade than the other portion of the image, and the change in shade is distinct, occurs along a line and is very visible to the human eye.
Prior attempts to increase the amount of ink which is applied to the ink-receptive area of the plate which prints the portion of the image which has the greatest circumferential extent around the cylinder have failed to solve the starvation problem. These efforts have involved adjustment of the inker including adjustment of the ink keys and changing the length and frequency of oscillation of vibrator rollers of the inker.
In accordance with the present invention, the aforementioned starvation problem is solved by providing the inker with an axially movable form roll. The axially movable form roll moves axially while it is transmitting ink to the plate on the plate cylinder. In this way, a circumferential section of the form roller which had been in contact with a nonink-receptive area of the plate moves to an ink-receptive area. This movement mates an area of nondepleted ink thickness on the form roll to an area of higher ink demand on the plate. This results in eliminating the very sharp distinct color change which is visible to the human eye. Unlike the conventional vibrator which moves only small thicknesses of ink laterally on the ink covered rolls, this invention moves large thicknesses of ink by moving the entire form roll.
In accordance with the preferred embodiment of the present invention, the inker includes three form rolls which are spaced circumferentially around the plate cylinder. The form roll, which is the last of the form rolls to be encountered by an area of the plate as it rotates, is the one that is axially movable. The axially movable form roll is mounted for rotation on a nonrotating central shaft. The form roll is also mounted for axial movement along the nonrotating shaft. The form roll is moved axially by its pressure engagement with a vibrating roll of the ink distribution system. Specifically, the axial movement of the vibrating roll causes axial movement of the form roll.
The form roll, however, is limited in its axial movement to an increment of the total axial movement of the vibrating roll. Specifically, the form roll will move axially small distances compared to that of the vibrating roll. As the vibrating roll moves axially from one extreme position, it will move the form roll axially a small increment. The form roll will then encounter a stop which limits axial further movement of the form roll with the vibrating roll. After the vibrating roll completes its total stroke, it starts to move in a reverse axial direction. The form roll will then move axially with the form roll an increment in the reverse direction. Thus, for a total cycle of movement of the vibrating roll, the form roll will move axially a predetermined distance, stop while the vibrating roll continues to move, then will move for the same predetermined distance in the reverse direction and stop while the vibrating roll continues to move to its initial or start position. This movement continues as long as printing occurs.
As a result of this incremental movement of the form roll, the amount of relative movement between the form roll and the printing plate is minimized. Thus, detrimental effects on the printing plate due to the relative movement of the form roll and printing plate are minimized. For example, if the form roll moved with the vibrating roll through its entire stroke, the ink-receptive and ink-rejecting areas, respectively, of the printing plate would be quickly worn. Thus, the life of the plate would be minimal. The axial movement of the form roll in accordance with the present invention is sufficient to eliminate the starvation problem but not sufficient enough to create significant wear of the printing plate.
Further, the inker of the present invention preferably includes a means for selectively preventing the axial movement of the form roll, if such movement is not required to eliminate the above discussed starvation problem for the image being printed.